{# empty array #}
{{ []|join(',') }}

{{ [1, 2]|join(',') }}
{{ ['foo', "bar"]|join(',') }}
{{ {0: 1, 'foo': 'bar'}|join(',') }}
{{ {0: 1, 'foo': 'bar'}|keys|join(',') }}

{# nested arrays #}
{% set a = [1, 2, [1, 2], {'foo': {'foo': 'bar'}}] %}
{{ a[2]|join(',') }}
{{ a[3]["foo"]|join(',') }}

{# works even if [] is used inside the array #}
{{ [foo[bar]]|join(',') }}

{# elements can be any expression #}
{{ ['foo'|upper, bar|upper, bar == foo]|join(',') }}

{# arrays can have a trailing , like in PHP #}
{{
  [
    1,
    2,
  ]|join(',')
}}
